Friday, July 8, 2022

The control centre

Even if you are not a science buff, having a better understanding of how sheep digest their food can enhance your stock and pasture management.

New Zealand’s pasture-based system is naturally high in proteins, which are required for animal growth (See table).

Lactating ewes need between 16-18% crude protein a day and only 9-12% at maintenance. Lambs grazing pasture need a minimum of 14% a day.

Proteins, often referred to as the building blocks to life, are made up from 20 standard amino acids. All amino acids contain nitrogen, hence the link between pasture and its need for nitrogen, which also helps explain why dung/urine patches are high in nitrogen as unused protein by the animal is excreted as urea.

As sheep graze the rumen converts plant protein into microbial protein, which the animal can then digest. In order to convert this plant protein, the rumen needs digestible energy – and current thinking in pasture systems is that digestible energy could be the limiting factors. In other words, if the animal had more digestible energy in its diet, would the rumen be able to convert more plant protein, in turn meaning increased growth?

This question has prompted the theory that sugar (carbohydrate) added to a sheep’s diet, or grazing of higher sugar ryegrasses may increase digestible energy, increasing microbial protein levels, enabling increased growth.

AgResarch farm system scientist David Stevens said, though, that research had shown only a small amount of the extra microbial protein produced would actually be used by an animal for growth. More nitrogen was excreted via dung, and away from urine, which did have a positive environmental impact.

Increased utilisation of protein would be beneficial if there was an increase in a select number of amino acids such as methionine, which can be in short supply in pasture and grain-dominant feed systems. Methionine is needed for meat and wool production in sheep.

Even without the additional supplementation of certain amino acids, Stevens said the key was in maintaining quality of pasture.

“In our grazing situation, the animal is eating and digesting reasonably constantly, so the protein from the current mouthful is being matched up with energy from the grass eaten several hours ago, so no real advantage occurs by adding extra energy at the time.

“There has been a bit of work done around trying to add sugar for better digestive synchrony (supplying the protein and the energy at the same time to the animal), but this hasn’t yielded any real advantages.”

Where a sheep’s diet can benefit from higher sugar is when feed quality is reduced, which restricts digestion and slows intake. Stevens said sheep grazing summer hill-country pastures or during drought could benefit from sugar supplementation such as grain or molasses because it helped the digestive process by making the rumen more efficient. In some circumstances protein supplementation might also be needed.

Animal performance can also become compromised if protein levels become too high and there is an absence of fibre. The acidosis that may occur is the combined effect of rapid increase in lactic acid build-up due to the low fibre (and, occasionally, high sugar) and build-up of ammonia (nitrogen) in the rumen, upsetting the pH level. This is especially the case if stock are introduced to a highly digestible, high-protein diet, such as rapidly growing Italian ryegrass, too quickly and the rumen microbes do not have time to adapt.

Lucerne crops and legume-rich specialist crops and, at times, pastures rich in clover, need hay or straw as a good source of digestible fibre to slow the rumen digestion, giving the rumen microbes time to convert the protein. Lambs often have a noticeable scour and struggle to gain weight if protein content is too high and there is no fibre supplementation.

Did you know?

  • Less ryegrass stem and the absence of dead matter mean lower non-digestible fibre, resulting in higher quality pasture
  •  Ryegrass seed heads and pasture dead matter slow the rumen down too much which lowers feed intake resulting in lower weight gains
  •  Sheep and cattle do benefit from protein supplementation when on a grain-rich diet. Grain is rich in carbohydrate (sugar) but low in protein
  •  Bypass protein, more commonly used as a supplement in the dairy industry, is a form of undegradable protein. This means it bypasses the rumen processes and can be absorbed by the small intestine and turned into glucose in the liver. It is used as an energy source at times of calving and early lactation stress
  •  Clover plants fix nitrogen from the air and soil but releases the nitrogen only as it dies, hence the reason new pastures tend to be nitrogen deficient while getting established.


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